Nearly every airbag, in particular every front-seat airbag, features a ventilation device through which gas from the gas compartment surrounding the outer cover of the airbag—hereinafter called the first gas compartment—can escape when it impinges on the person to be protected, so that the kinetic energy of the person to be protected is dissipated and said person will not be simply thrown back by the cover of the airbag. In the simplest case, such a ventilation device is simply a ventilation opening, in particular, a ventilation opening in the airbag cover.
In order to take account of different accident situations and/or of the weight of the person to be protected, adaptive ventilation devices are also known from prior art, which in addition to the ventilation opening also feature a throttle device, which, in a first state, throttles the gas flow through the ventilation opening more strongly than in the second state. Generally, the first state is the initial state, and the second state is the optional end state. A throttle device of this kind can in a simple case consist of a flap acting as throttle element and of a tear seam, so that the flap is sewn around the ventilation opening together the first cover: If the pressure in the first gas compartment exceeds a predefined value, then the tear seam bursts and the flap releases the ventilation opening.
It has also been known to provide an actuation unit, which in response to an external signal, in particular to an external electrical signal, changes the throttle device from its first state into its second state. For example, an actuator in the form of a pyrotechnic actuator is known from U.S. Pat. No. 6,648,371 B2. In this case, the first end of a strap is connected to the throttle element and the second end of said strap is connected to a bolt of the actuator. The actuator in this case is held, in particular, on the housing bottom of the airbag module. As long as the actuator is not activated, the throttle element is thus connected via the strap to the housing bottom. If the airbag cover is fully expanded, then the strap moves into a tensioned state which holds the throttle element in its first state. If the actuator is operated, then the bolt which holds the second end of the pull element, is blasted off by a pyrotechnic charge, the pull element loses its tension and the throttle element changes to its second state, namely its unthrottled state. This technology basically functions very well; it has, however, the disadvantage that some parts, such as the bolt, for example, can fly around inside the gas compartment of the airbag cover, and that open flames are generated by the pyrotechnic charge inside the airbag cover. Furthermore, the pyrotechnic charge injects additional gas into the gas compartment of the airbag cover, which is not always desirable.
A throttle element in the form of a nozzle extending from a first to a second end is known from US 2006/0071461 A1. In this regard, the first end around the ventilation opening is secured to the cover of the airbag and the second end features a tunnel through which a first end of a strap extends. In the initial state, the second end of the strap is connected to an actuator, for example to one like that described above. In the initial state (hereinafter usually designated as the first state), the nozzle is located inside the gas compartment surrounding the cover of the airbag. If this airbag is filled by its associated inflator, which causes the airbag cover to expand, then the strap is subjected to tensile stress and pulls tight the second end of the nozzle, as a result of which the ventilation device formed by the ventilation opening and nozzle is throttled. If the actuator is now operated, so that the second end of the strap is released, then the nozzle is pulled outwardly through the ventilation opening due to the overpressure prevailing in the first gas compartment. The strap is now no longer under tensile stress and can no longer hold the second end of the nozzle, so that the ventilation device changes to an unthrottled, second state.
The generic specification WO2011/157631 A1 proposes an airbag module with an adaptive ventilation device in which a second, small airbag is disposed inside the gas compartment of the actual airbag cover and acts as a part of an actuator. The cover of this second airbag—hereinafter designated as the second cover—surrounds a second gas compartment which can be filled with gas by a gas generator in the form of an ignition capsule. If this occurs, then this influences the free cross-section of the ventilation opening. In the embodiment known from WO2011/157631 A1, the second cover is sewn at least in sections around the ventilation opening together with the first cover by means of a tear seam. This means that, in the initial state, the second cover covers the ventilation opening and thus seals it. When the second cover expands, this tear seam tears apart and the ventilation opening is released. The second cover in this case has a dual function: Firstly, it serves as a part of the actuator, and secondly, as a part of the throttle device, namely as throttle element. The disadvantage of the described system is that the tear seam must withstand the forces acting on the second cover used as a cover element due to the internal pressure occurring in the first gas compartment, but otherwise must break apart owing to the forces occurring as a result of filling the second cover. This is difficult to adjust.
Proceeding from this point, the object of the present invention is to improve a generic airbag module such that it can be reliably ensured with simple means that the tear seam remains intact as long as the second cover is not filled with gas and breaks apart when the second cover is filled with gas.
This object is attained by an airbag module as described herein.